does not occur selectively to any component of the alloy.If this
is not true,then the calculation approach will need to be
adjusted to reflect the observed mechanism.In addition,some
rationale must be adopted for assigning values of n to the
elements in the alloy because many elements exhibit more than
one valence value.
4.4To calculate the alloy equivalent weight,the following
approach may be used.Consider a unit mass of alloy oxidized.
The electron equivalent for 1g of an alloy,Q is then:
Q 5(nifiWi (3)
where:fi5the mass fraction of the i th element in the alloy,Wi 5the atomic weight of the i th element in the alloy,and ni 5the valence of the i th element of the alloy.
Therefore,the alloy equivalent weight,EW ,is the reciprocal
of this quantity:
EW 51(nifiWi (4)Normally only elements above 1mass percent in the alloy
are included in the calculation.In cases where the actual
analysis of an alloy is not available,it is conventional to use the
mid-range of the composition specification for each element,unless a better basis is available.A sample calculation is given in Appendix X2(1).44.5Valence assignments for elements that exhibit mul
tiple valences can create uncertainty.It is best if an independent technique can be used to establish the proper valence for each alloying element.Sometimes it is possible to analyze the corrosion products and use those results to establish the proper valence.Another approach is to measure or estimate the electrode potential of the corroding surface.Equilibrium dia-grams showing regions of stability of various phases as a function of potential and pH may be created from thermody-namic data.These diagrams are known as Potential-pH (Pour-baix)diagrams and have been published by several authors (2,3).The appropriate diagrams for the various alloying elements can be consulted to estimate the stable valence of each element at the temperature,potential,and pH of the contacting electro-lyte that existed during the test.N OTE 2—Some of the older publications used inaccurate thermody-namic data to construct the diagrams and consequently they are in error.
4.6Some typical values of EW for a variety of metals and alloys are given in Table 1.TABLE 1Equivalent Weight Values for a Variety of Metals and Alloys
Common
Designation UNS Elements w/Constant
Valence
Lowest Second Third Fourth Variable Valence Equivalent Weight Variable Valence Equivalent Weight Element/Valence Equivalent Weight Element/Valence Equivalent Weight Aluminum Alloys:AA1100A A91100Al/3
8.99AA2024A92024Al/3,Mg/2
Cu/19.38Cu/29.32AA2219A92219Al/3
Cu/19.51Cu/29.42AA3003A93003Al/3
Mn/29.07Mn/49.03Mn 78.98AA3004A93004Al/3,Mg/2
Mn/29.09Mn/49.06Mn 79.00AA5005A95005Al/3,Mg/2
9.01AA5050A95050Al/3,Mg/2
9.03AA5052A95052Al/3,Mg/2
9.05AA5083A95083Al/3,Mg/2
9.09AA5086A95086Al/3,Mg/2
9.09AA5154A95154Al/3,Mg/2
9.08AA5454A95454Al/3,Mg/2
9.06AA5456A95456Al/3,Mg/2
9.11AA6061A96061Al/3,Mg/2
9.01AA6070A96070Al/3,Mg/2,
Si/4
8.98AA6101A96161Al/3
8.99AA7072A97072Al/3,Zn/2
9.06AA7075A97075Al/3,Zn/2,
Mg/2
Cu/19.58Cu/29.55
AA7079A97079Al/3,Zn/2,
Mg/2
9.37AA7178A97178Al/3,Zn/2,
Mg/2Cu/19.71Cu/29.68Copper Alloys:
CDA110
C11000Cu/163.55Cu/231.77CDA220
C22000Zn/2Cu/158.07Cu/231.86CDA230
C23000Zn/2Cu/155.65Cu/231.91CDA260
C26000Zn/2Cu/149.51Cu/232.04CDA280
C28000Zn/2Cu/146.44Cu/232.11CDA444
C44300Zn/2Cu/1,Sn/250.42Cu/1,Sn/450.00Cu/2,Sn/432.00CDA687
C68700Zn/2,Al/3Cu/148.03Cu/230.29CDA608
C60800Al/3Cu/147.114Cu/227.76CDA510
C51000Cu/1,Sn/263.32Cu/1,Sn/460.11Cu/2,Sn/431.66CDA524C52400Cu/1,Sn/263.10Cu/1,Sn/457.04Cu/2,Sn/431.55
4
The boldface numbers in parentheses refer to the list of references at the end of
this
standard.
TABLE1Continued
Common Designation UNS
Elements
w/Constant
Valence
Lowest Second Third Fourth
Variable
Valence
Equivalent
Weight
Variable
Valence
Equivalent
Weight
Element/
Valence
Equivalent
Weight
Element/
Valence
Equivalent
Weight
CDA655C65500Si/4Cu/150.21Cu/228.51
CDA706C70600Ni/2Cu/156.92Cu/231.51
液氨化工厂制备CDA715C71500Ni/2Cu/146.69Cu/230.98 CDA752C75200Ni/2,Zn/2Cu/146.38Cu/231.46
Stainless Steels:
304S30400Ni/2Fe/2,Cr/325.12Fe/3,Cr/318.99Fe/3,Cr/615.72
321S32100Ni/2Fe/2,Cr/325.13Fe/3,Cr/319.08Fe/3,Cr/615.78
309S30900Ni/2Fe/2,Cr/324.62Fe/3,Cr/319.24Fe/3,Cr/615.33
310S31000Ni/2Fe/2,Cr/324.44Fe/3,Cr/319.73Fe/3,Cr/615.36
316S31600Ni/2Fe/2,Cr/3,Mo/325.50Fe/2,Cr/3,Mo/425.33Fe/3,Cr/6,Mo/619.14Fe/3,Cr/6,Mo/616.111 317S31700Ni/2Fe/2,Cr/3,Mo/325.26Fe/2,Cr/3,Mo/425.03Fe/3,Cr/3,Mo/619.15Fe/3,Cr/6,Mo/615.82 410S41000Fe/2,Cr/325.94Fe/3,Cr/318.45Fe/3,Cr/616.28
430S43000Fe/2,Cr/325.30Fe/3,Cr/318.38Fe/3,Cr/615.58
446S44600Fe/2,Cr/324.22Fe/3,Cr/318.28Fe/3,Cr/614.46
20CB3A N08020Ni/2Fe/2,Cr/3,Mo/3,
Cu/123.98Fe/2,Cr/3,Mo/
4,Cu/1
23.83Fe/3,Cr/3,Mo/
6,Cu/2
18.88Fe/3,Cr/6,Mo/6,
Cu/2
15.50
Nickel Alloys:
200N02200NI/229.36Ni/319.57
400N04400Ni/2Cu/135.82Cu/230.12
600N06600Ni/2Fe/2,Cr/326.41Fe/3,Cr/325.44Fe/3,Cr/620.73 800N08800Ni/2Fe/2,Cr/325.10Fe/3,Cr/320.76Fe/3,Cr/616.59
825N08825Ni/2Fe/2,Cr/3,Mo/3,
Cu/125.52Fe/2,Cr/3,Mo/
4,Cu/1
车载卫生间
25.32Fe/3,Cr/3,Mo/
6,Cu/2
21.70Fe/3,Cr/6,Mo/6,
Cu/2
17.10
B N10001Ni/2Mo/3,Fe/230.05Mo/4,Fe/227.50Mo/6,Fe/223.52Mo/6,Fe/323.23
C-22B N06022Ni/2Fe/2,Cr/3,Mo/3,
W/426.04Fe/2,Cr/3,Mo/
4,W/4
25.12Fe/2,Cr/3,Mo/
6,W/6
23.28Fe/3,Cr/6,Mo/6,
W/6
17.88
C-276N10276Ni/2Fe/2,Cr/3,Mo/3,
W/427.09Cr/3,Mo/425.90Fe/2,Cr/3,Mo/
6,W/6
23.63Fe/3,Cr/6,Mo/6,
W/6
19.14
G N06007Ni/2(1)25.46(2)22.22(3)22.04(4)17.03 Carbon Steel:Fe/227.92Fe/318.62
(1)5Fe/2,Cr/3,Mo/3,Cu/1,Nb/4,
Mn/2
(3)5Fe/3,Cr/3,Mo/6,Cu/2,Nb/5,Mn/2
(2)5Fe/2,Cr/3,Mo/4,Cu/2,Nb/5,
Mn/2
(4)5Fe/3,Cr/6,Mo/6,Cu/2,Nb/5,Mn/4
Other Metals:
Mg M14142Mg/212.15
Mo R03600Mo/331.98Mo/423.98Mo/615.99
Ag P07016Ag/1107.87Ag/253.93
Ta R05210Ta/536.19
Sn L13002Sn/259.34Sn/429.67
Ti R50400Ti/223.95Ti/315.97Ti/411.98
Zn Z19001Zn/232.68
Zr R60701Zr/422.80
Pb L50045Pb/2103.59Pb/451.80
A Registered trademark Carpenter Technology.
B Registered trademark Haynes International.
N OTE1—Alloying elements at concentrations below1%by mass were not included in the calculation,for example,they were considered part of the basis metal. N OTE2—Mid-range values were assumed for concentrations of alloying elements.
N OTE3—Only consistent valence groupings were used.
N OTE4—(Eq4)was used to make these calculations.
4.7Calculation of Corrosion Rate—Faraday’s Law can be
used to calculate the corrosion rate,either in terms of penetra-
tion rate(CR)or mass loss rate(MR)(4):
CR5K1i cor
r EW(5)
MR5K2i cor EW(6) where:
CR is given in mm/yr,i cor inµA/cm2,
K153.27310−3,mm g/µA cm yr(Note3),
r5density in g/cm3,(see Practice G1for density values for many metals and alloys used in corrosion test-
ing),
MR5g/m2d,and
K258.954310−3,g cm2/µA m2d(Note3).
N OTE3—EW is considered dimensionless in these calculations. Other values for K1and K2for different unit systems are given in Table2.
4.8Errors that may arise from this procedure are discussed below.
4.8.1Assignment of incorrect valence values may cause serious errors(5).
4.8.2The calculation of penetration or mass loss from electrochemical measurements,as described in this standard, assumes that uniform corrosion is occurring.In cases where non-uniform corrosion processes are occurring,the use of
these
methods may result in a substantial underestimation of the true values.
4.8.3Alloys that include large quantities of metalloids or oxidized materials may not be able to be treated by the above procedure.
4.8.4Corrosion rates calculated by the method above where abrasion or erosion is a significant contributor to the metal loss process may yield significant underestimation of the metal loss rate.
5.Polarization Resistance
5.1Polarization resistance values may be approximated from either potentiodynamic measurements near the corrosion potential (see Practice G 59)or stepwise potentiostatic polar-ization using a single small potential step,D E ,usually either 10mV or −10mV ,(see Test Method D 2776).Values of 65and 620mV are also commonly used.In this case,the specimen current,D I ,is measured after steady state occurs,and D E/D I is calculated.Potentiodynamic measurements yield curves of I versus E and the reciprocal of the slope of the curve (dE/dI)at the corrosion potential is measured.In most programmable potentiodynamic polarization equipment,the current is con-verted to current density automatically and the resulting plot is of i versus E .In this case,the polarization resistance is given by dE/di at the corrosion potential and 5.2is not applicable.
5.2It is necessary to multiply the dE/dI or D E/D I value calculated above by the exposed specimen g
eometric area to obtain the polarization resistance.This is equivalent to the calculation shown in 4.1for current density.
5.3The Stern-Geary constant B must be estimated or calculated to convert polarization resistance values to corrosion current density (6,8).
5.3.1Calculate Stern-Geary constants from known Tafel slopes where both cathodic and anodic reactions are activation controlled,that is,there are distinct linear regions near the corrosion potential on an E log i plot:
B 5ba bc
2.303~ba 1bc !(7)
where:ba 5slope of the anodic Tafel reaction,when plotted on
base 10logarithmic paper in V/decade,bc 5slope of the cathodic Tafel reaction when plotted on
base 10logarithmic paper in V/decade,and B 5Stern-Geary constant,V .5.3.2In cases where one of the reactions is purely diffusion controlled,the Stern-Geary constant may be calculated:B 5b 2.303(8)
麦克力电气where:b 5the activation controlled Tafel slope in V/decade.5.3.3It should be noted in this case that the corrosion current density will be equal to the diffusion limited current density.A sample calculation is given in Appendix X4.5.3.4Cases where both activation and diffusion effects are similar in magnitude are known as mixed control.The reaction under mixed control will have an apparently larger b value than predicted for an activation control,and a plot of E versus log I will tend to curve to an asymptote parallel to the potential axis.The estimation of a B value for situations involving mixed control requires more information in general and is beyond the scope of this standard.In general,Eq 7and Eq 8may be used,and the corrosion rate calculated by these two approximations may be used as lower and upper limits of the true rate.N OTE 4—Electrodes exhibiting stable passivity will behave as if the anodic reaction were diffusion limited,except that the passive current density is not affected by agitation.5.3.5It is possible to estimate b a and b c from the deviation from linearity of polarization curves in the 20–50mV region around the corrosion potential.Several approaches have been proposed based on analyses of electrode kinetic models.See Refs (9-11)for more information.5.3.6In cases where the reaction mechanism is known in detail,the Tafel slopes may be estimated from the rate controlling step in the mechanism of the reaction.In general,Tafel slopes are given by (12):b 5KRT nF (9)where:K 5a constant,R 5the perfect gas constant,T 5the absolute temperature,n 5the number of electrons involved in the reaction step,and F 5Faraday’s constant.At 25°C,(RT 2.303F )is
59.2mV/decade.For simple one electron reactions,K is usually found to be 2.0.5.3.7In cases where the Tafel slopes cannot be obtained from any of the methods described above,it may be necessary to determine the Stern-Geary constant experimentally by measuring mass loss and polarization resistance values.5.4The corrosion current density may be calculated from the polarization resistance and the Stern-Geary constant as follows:i cor 5B R p (10)The corrosion rate may then be calculated from the corrosion current,as described in Section 4.A sample calculation is given
in Appendix X5.
TABLE 2Values of Constants for Use in Faraday’s Equation Rate
A
Penetration
Rate Unit (CR)I cor Unit r Unit K 1Units of K 1A
mpy µA/cm 2g/cm 30.1288mpy g/µA cm
mm/yr B A/m 2B kg/m 3B 327.2mm kg/A m y
mm/yr B µA/cm 2g/cm 3 3.27310−3mm g/µA cm y
B
Mass Loss Rate
Unit I cor Unit K 2Units of K 2A
g/m d A/m 0.8953g/Ad
mg/dm 2d (mdd)µA/cm 20.0895mg cm 2/µA dm 2d
mg/dm 2d (mdd)A/m 2B 8.953310−3mg m 2/A dm 2d固体破碎机
A EW is assumed to be dimensionless.
B SI
unit.
5.5There are several sources of errors in polarization resistance measurements:
工艺品加工
设备5.5.1Solution resistivity effects increase the apparent polar-ization resistance,whether measured by the potentiostatic or potentiodynamic methods(13).The effect of solution resis-tance is a function of the cell geometry,but the following expression may be used to approximate its magnitude.
R p5R a2r l(11) where:
R a5the apparent polarization resistance,ohm cm2,
r5the electrolyte resistivity in ohm cm,
l5the distance between the specimen electrode and the Luggin probe tip,or the reference electrode in cm,and R p5the true polarization resistance in ohm cm2.
Significant solution resistivity effects cause the corrosion rate to be underestimated.A sample calculation is given in Appendix X6.
5.5.2Potentiodynamic techniques introduce an additional error from capacitative charging effects.In this case,the magnitude of the error is proportional to scan rate.The error is
illustrated by(Eq12):
I total5I f1c S dV dt D(12)
where:
I total5the cell current,
I f5the Faradaic current associated with anodic and
cathodic processes,
c5the electrode capacitance,and
dV/dt5the scan rate.
The capacitance charging effect will cause the calculated
polarization resistance to be in error.Generally,this error is
small with modest scan rates(14).
5.5.3Corroding electrodes may be the site for other elec-
trochemical reactions.In cases where the corrosion potential is
within50to100mV of the reversible potential of the corroding
electrode,the electrochemical reactions will occur simulta-
neously on the electrode surface.This will cause either the
anodic or cathodic b value to appear smaller than the corrosion
reaction above.Consequently,the Stern-Geary constant B will
be inflated and the predicted corrosion current will be overes-
timated(15).In this case,the concentration of the corroding
electrode ions is generally of the same magnitude or higher
than other ions participating in the corrosion process in the漆雾净化
装置
electrolyte surrounding the electrode.Other redox couples that
do not necessarily participate in the corrosion reaction may
have similar effects.This is especially true for metals exhibit-
ing passive behavior.
6.Keywords
6.1corrosion current;corrosion rate;electrochemical;
equivalent weight;polarization resistance;Tafel slopes
APPENDIXES
(Nonmandatory Information)
X1.SAMPLE CALCULATION—CORROSION CURRENT DENSITY
X1.1Data:
X1.1.1Corrosion Current—27.0µA.
X1.1.2Specimen Size—round anode area exposed. X1.1.3Diameter—1.30cm.X1.2Calculation—See(Eq1)in text:
i cor5
27.0
~1.30!2
p
4
5
27.0
1.32
520.3µA/cm2(X1.1)
X2.SAMPLE CALCULATION—ALLOY EQUIV ALENT WEIGHT
X2.1Data:
X2.1.1Alloy—UNS S31600,actual composition not avail-able.
X2.1.2Corrosion Potential—300mV versus SCE1N sul-furic acid.
X2.2Assumptions:
X2.2.1Composition:
X2.2.1.1Chromium16–18%—mid range17%.
X2.2.1.2Nickel—10–14%—mid range12%.
X2.2.1.3Molybdenum—2–3%—mid range2.5%.
X2.2.1.4Iron—Balance(ignore minor elements).
1711212.5531.5(X2.1)
X2.2.1.5Iron5100−31.5568.5%.
X2.2.2Valence values from Ref(2).
Chromium—+3
Nickel—+2
Molybdenum—+3
Iron—+2
X2.3Calculations—For simplicity,assume100g of alloy dissolved.Therefore,the gram equivalents of the dissolved components are given by(Eq3).
Q5
17
51.996
331
12
58.71
321
2.5
95.94
331
68.5
55.847
32
(X2.2)
50.98110.40910.07812.45353.921g equivalents
The alloy equivalent weight is therefore100⁄3.9215
25.50.
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